TBE NATURAL HISTORV OF CLOUDS. Ap 



But, though ihe chemical action of air is imperceptible, Mechanical ef- 

 its ffitfc/mtticfl/ effect is great. A movincr atmosphere may ^^<^'°^^" °" 

 double or triple the rate ol evaporation, according to its v-e- 

 locity. For not wily is tfie surface, from which only the 

 vapour escapes, thus enlarged and changed ; but the nas- 

 cent vapour itself, which would otherwise hover a while upon 

 it, to the obstruction of the process, is immediately brushed 

 away and diffused. 



By applying these principles, we may explain to ourselves Explanation of 

 various natural phenomena: as for instance ; why the wind, '^^'^'^"^ P^'®"'*' 

 after rain, becomes colder than even the rain which fell ; 

 being robbed ol it'^ caloric by the evaporation of the floating 

 and deposited water, with which it is in contact: why snow 

 sdmetimes totally disappears without melting, and the sur- 

 face' of ice becomes sensibly wasted and channelled; for 

 these are warm, compared with the dry and frosty air which 

 blows at such times, and consequently evaporate freely. In 

 ;what manner, again, a strong westerly wind in summer or 

 autumn brings up clouds, which on its cessation descend in 

 rsin: for it promotes evaporation by its mechanical effect, 

 and the vapour escapes into an atmosphere already too 

 moist to carry it off to any great distance. This will be 

 evident by recurring to the principle before stated, that the 

 vapour escapes by the force of the temperature of the water 

 out of which it is formed; and, consequently, into a colder 

 ivtmosphere it will still escape, though continually decom- 

 posed thereby. 



Vapouris decomposed by air, in consequence of the su- Decomposi- 

 .perior affinity of the latter to caloric. This happens in two ^^°^ of t^pour. 

 ways. 1. When vapour escapes or is propelled into air 

 .colder than itself; the result being a local dense cloud. 2. 

 ,When a mixture of air and vapour is cooled ; in which case 

 there ensues a general turbiness, which we shall exclusively 

 iflenominate haze. It is occasioned by minute floating par- H*«e. 

 •tides of water ; the caloric which, united to these, formed 

 transparent vapour, having passed into the air. 



Out of this haze clouds may be afterwards formed, by 

 -simple aggregation, or by electrical attraction. It abounds 

 in the atmosphere during the most part of the year, occupy- 

 in<^. sometimes the higher, sometiroesthelower, part thereof. 



"voL. XXX.— Sept. 1811. E ' Th« 



